Cerium citrate, method of making and corrosion inhibitor comprising cerium citrate

ABSTRACT

A method of making cerium citrate includes combining cerium carbonate and citric acid to produce cerium citrate and carbon dioxide. The cerium citrate is substantially free of negative ions other than citrate. The cerium citrate can be used in a corrosion inhibitor composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/173,847 filed Jun. 6, 2016 which incorporated by referenceherein in its entirety.

BACKGROUND

This disclosure relates to corrosion inhibitors and, more particularly,to methods of making corrosion inhibitors.

Components made from metallic alloys, such as aluminum alloys, achievehigher strengths through inclusion of alloying elements. However, thepresence of these alloying elements tends to make the alloy vulnerableto corrosion. Typically, the component utilizes a protective coatingcontaining a corrosion-inhibitor to protect the underlying alloy fromcorrosion.

One type of corrosion-inhibitor includes hexavalent chromium in the formof a barium or strontium chromate compound, for example. Althougheffective, hexavalent chromium is commonly recognized as a carcinogenand is therefore undesirable for use as a coating.

Chrome-free corrosion-inhibitors have been used as an alternative tohexavalent chromium inhibitors. For example, chrome-free corrosioninhibitors utilize anodic and cathodic corrosion inhibitors to resistcorrosion of the underlying alloy.

The effectiveness of the anodic and cathodic corrosion inhibitors isrelated to their composition and availability for corrosion inhibition.Accordingly, it is desirable to provide improved methods for thesynthesis of corrosion inhibitors to improve the quality andavailability of the corrosion inhibitor.

SUMMARY

In some aspects of the disclosure, a method of making cerium citrateincludes combining cerium carbonate and citric acid to produce ceriumcitrate and carbon dioxide.

In the method described above the cerium carbonate may be a hydratedcerium carbonate.

In any of the foregoing embodiments, the cerium carbonate and citricacid may be combined in the presence of a solvent.

In any of the foregoing embodiments, the method may further compriseswashing the cerium citrate, drying the washed cerium citrate andreducing the dried cerium citrate to a fine powder.

In any of the foregoing embodiments, the cerium citrate may have greaterthan or equal to 5 weight percent water of hydration.

In any of the foregoing embodiments, the cerium citrate may be atrivalent cerium citrate.

In any of the foregoing embodiments, the cerium carbonate and citricacid may be subjected to high energy ball milling. The high energy ballmilling may be performed using stainless steel, ceramic, or tungstenballs. The cerium carbonate may be a hydrate.

In another embodiment, a cerium citrate produced by combining ceriumcarbonate and citric acid is described and the cerium citrate issubstantially free of negative ions other than citrate. The ceriumcitrate may be hydrated. When hydrated, the cerium citrate may havegreater than or equal to 10 weight percent water of hydration. In any ofthe foregoing embodiments the cerium citrate may be a trivalent ceriumcitrate.

In another embodiment, a corrosion inhibitor composition includes acerium citrate substantially free of negative ions other than citrate.The cerium citrate may be a hydrate. When the cerium citrate is ahydrate it may have greater than or equal to 10 weight percent water ofhydration. In any of the foregoing embodiments the cerium citrate may bea trivalent cerium citrate. In any of the foregoing embodiments thecorrosion inhibitor composition may include zinc molybdate. Thecorrosion inhibitor composition of any of the foregoing embodiments maybe combined with at least one of an adhesive, primer, paint, lubricant,cooling fluid, sealant and epoxy.

Also described herein is a metal substrate and a corrosion inhibitordisposed on the substrate, wherein the corrosion inhibitor includes acerium citrate substantially free of negative ions other than citrate.

The foregoing features and elements may be combined in variouscombinations without exclusivity, unless expressly indicated otherwise.These features and elements as well as the operation thereof will becomemore apparent in light of the following description and the accompanyingdrawings. It should be understood, however, that the followingdescription and drawings are intended to be illustrative and explanatoryin nature and non-limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure isparticularly pointed out and distinctly claimed in the claims at theconclusion of the specification. The foregoing and other features, andadvantages of the present disclosure are apparent from the followingdetailed description taken in conjunction with the accompanying drawingin which:

FIG. 1 shows the x-ray diffraction patterns for cerium citratesynthesized from cerium carbonate, cerium nitrate and cerium chloride.

DETAILED DESCRIPTION

Cerium citrate is a cathodic corrosion inhibitor for aluminum alloys.Surprisingly, it has been found that the method of synthesis of thecerium citrate can affect its effectiveness as a corrosion inhibitor.Typically, cerium citrate has been made using cerium chloride or ceriumnitrate as a starting material. Unfortunately, the negative ioncomponents of these compounds, the chloride and the nitrate, whenreleased during the synthesis of cerium citrate can adsorb to the ceriumcitrate surface. These negative ions are then included in the corrosioninhibitor composition and can harm the corrosion protection due to ionicmobility and result in corrosion issues. As a result of this discovery anew method of cerium citrate synthesis was required, one in which theissues of negative ion adsorption would be avoided.

Disclosed herein is a method of making cerium citrate comprisingreacting cerium carbonate hydrate with citric acid as shown in formula(1)Ce₂(CO₃)₃·mH₂O+2C₆H₈O₇→2Ce(C₆H₅O₇)+3CO₂+(m+3)H₂O  (1)Advantageously a product of the reaction is carbon dioxide which is notionic and is typically a gas under the reaction conditions. Hence, thecarbon dioxide product is not adsorbed to the surface of the ceriumcitrate and the cerium citrate produced in this manner does not comprisenegative ions that affect the corrosion protection in a negative manner.Additionally, it should be noted that the cerium citrate produced usingcerium carbonate as a starting material has a different crystalstructure than cerium citrate produced using either cerium nitrate orcerium chloride as a starting material. The cerium carbonate hydrateshown in formula (1) has a hydration level m and m can have a value of 0to 10, or, 1 to 9, or, 2 to 8. The cerium citrate produced by thereaction can be a trivalent cerium citrate.

The reaction described by formula (1) can be performed by combiningcerium carbonate hydrate, citric acid and a solvent in an inertatmosphere. The cerium carbonate hydrate and citric acid are combined ina molar ratio of 1:1 to 1:3, or 1:2 to 1:2.5, or 1:2.1. Useful solventsinclude water and polar protic solvents having a low boiling point suchas methanol, ethanol, and propanol. The inert atmosphere typicallycomprises nitrogen or argon or other inert gas. The inert gas isemployed to prevent or limit oxidation of the cerium ion during ceriumcitrate synthesis. Carbon dioxide is not a useful gas for the inertatmosphere as it would impede or slow the progress of the reaction. Thereaction may be performed at a temperature of 50 to 90° C. for 5 to 7hours. Within this range the temperature can be 60 to 80° C. Thereaction time can be 5.5 to 6.5 hours. At this point the reaction mayhave a pH of 1 to 3. The reaction is then cooled to room temperature.After the reaction is cooled to room temperature additional ceriumcarbonate hydrate can be added. The reaction is continued for another 8to 12 hours. At this point the reaction may have a pH of 6 to 7.

The reaction product, cerium citrate, can be isolated by a solidseparation method such as filtration, centrifugation, or vacuumevaporation. Following separation the product is typically washed withdeionized water to remove any remaining citric acid. After washing thecerium citrate can be dried. Drying can occur at a temperature of 50° C.to 85° C. for 4 to 12 hours. In some embodiments a vacuum oven is used.Drying is not intended to remove all waters of hydration from the ceriumcitrate. In some embodiments the cerium citrate has greater than orequal to 5 weight percent water of hydration, or greater than or equalto 10 weight percent water of hydration. When the water of hydration isless than 10 weight percent, the corrosion inhibiting performance of thecerium citrate may be decreased. After drying, the resulting materialcan be reduced in size to a fine powder by milling, grinding or acombination thereof.

The reaction described by formula (1) can also be performed using highenergy ball milling. The cerium carbonate and citric acid can becombined in amounts as described above. The cerium carbonate ispreferably a hydrate although a hydrated cerium carbonate is notrequired. The reactants are combined in a container, typically stainlesssteel or tungsten lined, with stainless steel, ceramic or tungsten ballsof the desired number and size. The number and size of the balls istypically dictated by the size of the container and the amount ofmaterial being reacted. For example, 2.88 grams of cerium carbonatehydrate and 1.92 grams of citric acid can be combined with about a dozenof ceramic balls having a diameter of 0.25 inches in a cylindricalstainless steel autoclave (1.5 inch inner diameter, 2.25 inch in height)having a volume of 65 milliliters with a pressure relief valve. Thecontainer is purged with an inert gas such as nitrogen or argon, thenplaced in a shaker mill or similar device and agitated at a speed of 30to 720 rotations per minute (rpm). If the cerium carbonate employed isnot a hydrate then the reaction may be facilitated by the addition of asmall amount of solvent or water, for example, 10 to 1000 microliters ofwater per gram of cerium carbonate. Agitation continues forapproximately 60 hours. After the reaction is complete the product iswashed to remove any excess citric acid, dried and reduced to a finepowder as described above.

The cerium citrate produced as described herein is substantially free ofnegative ions other than citrate. Substantially free, as used in thiscontext, is defined as containing less than 1 weight percent (wt %)based on the total weight of the material, or, less than 0.1 wt % of anegative ion other than citrate.

The cerium citrate substantially free of negative ions other thancitrate can be used in a corrosion inhibitor composition. The corrosioninhibitor composition can further comprise anodic corrosion inhibitorssuch as molybdates, permanganates, tungstates, and/or vanadates. In someembodiments the corrosion inhibitor composition further comprises zincmolybdate.

The corrosion inhibitor composition may be included in adhesives, paintsand primers, organic sealants, epoxies, cooling fluids, lubricants, andthe like (hereafter referred to as a carrier fluid). These products maybe applied to a metal substrate that is being protected by any suitablemanner such as spraying, brushing, dipping, or the like. In addition,the corrosion inhibitor composition may be dissolved in a carrier suchas alcohol, water or the like and formed on the surface of a substrateas a conversion coating.

The corrosion inhibitor composition is particularly useful in preventinggeneral corrosion and pitting corrosion on metal substrates,particularly, high strength aluminum alloys for use in the aerospaceindustry. The corrosion inhibitor composition may be applied in anymanner known in the art including as a conversion coating, or applied asa primer, adhesive, epoxy, paint, organic sealant, sealer for anodizedaluminum, additive for recirculating water system or the like.

Examples

Cerium citrate samples synthesized from cerium carbonate, cerium nitrateand cerium chloride using wet chemistry were compared using x-raydiffraction (XRD) as shown in FIG. 1 . Cerium citrate synthesized fromcarbonate has one type of powder diffraction structure (type I), whichis different from cerium citrate synthesized from nitrate or chloride(which have type II structure). The cerium citrate synthesized fromthree different starting materials was each combined with an anodiccorrosion inhibitor. The corrosion inhibiting properties were evaluatedby comparing the corrosion current (in milliamperes per squarecentimeter) in an aqueous solution comprising 350 ppm NaCl. Lowercorrosion current indicates better corrosion inhibiting performance.Results are shown in Table 1.

TABLE 1 XRD Corrosion Current (mA/cm²) structure in 350 ppm NaCl Ceriumcitrate from carbonate Type I 0.02-0.08 Cerium citrate from chlorideType II 0.2-0.3 Cerium citrate from nitrate Type II 0.03-0.04

The effect of hydrated water was also examined. Cerium citrate wassynthesized from cerium carbonate and had differing levels of hydration.1.92 grams of citric acid was dissolved in 50 milliliters of deionizedwater. The mixture was heated to 80° C. 2.88 grams of cerium carbonatehydrate was added slowly over an hour. The reaction was continued for 6hours and the pH of the reaction mixture was 1-3 (by pH paper). Thereaction mixture was cooled and an additional 0.288 grams of ceriumcarbonate was added slowly and allowed to react overnight. The pH of thereaction mixture was 6-7 at the end of the overnight period. Thematerial was filtered, washed with deionized water and dried in an oven.One sample dried at 60° C. overnight had greater than 10 wt % waterwhereas one sample dried at 90° C. overnight had less than 10 wt %water. The cerium citrate with differing water content was each combinedwith the same anodic corrosion inhibitor. The corrosion inhibitingproperties were evaluated by comparing the corrosion current (inmilliamperes per square centimeter) in an aqueous solution comprising350 ppm NaCl. Lower corrosion current indicates better corrosioninhibiting performance. Results are shown in Table 2.

TABLE 2 Water Corrosion Current (mA/cm²) content in 350 ppm NaCl Ceriumcitrate from carbonate <10 wt % 1.0-1.3 Cerium citrate fromcarbonate >10 wt % 0.1-0.3

The use of the terms “a,” “an,” “the,” and similar references in thecontext of description (especially in the context of the followingclaims) are to be construed to cover both the singular and the plural,unless otherwise indicated herein or specifically contradicted bycontext. All ranges disclosed herein are inclusive of the endpoints, andthe endpoints are independently combinable with each other.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions,combinations, sub-combinations, or equivalent arrangements notheretofore described, but which are commensurate with the scope of thepresent disclosure. Additionally, while various embodiments of thepresent disclosure have been described, it is to be understood thataspects of the present disclosure may include only some of the describedembodiments.

Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

What is claimed is:
 1. A corrosion inhibitor composition comprising: an anodic corrosion inhibitor comprising a permanganate, a tungstate and/or a vanadate; and a cerium citrate containing less than 0.1 weight percent chloride and nitrate, has the formula Ce(C₆H₅O₇), has greater than or equal to 10 weight percent water of hydration, and is synthesized by combining cerium carbonate and citric acid to produce cerium citrate and carbon dioxide.
 2. The corrosion inhibitor composition of claim 1 in combination with at least one of an adhesive, primer, paint, lubricant, cooling fluid, sealant and epoxy.
 3. An article comprising: a metal substrate; and a corrosion inhibitor disposed on the substrate, wherein the corrosion inhibitor comprises: an anodic corrosion inhibitor comprising a permanganate, a tungstate and/or a vanadate; and a cerium citrate containing less than 0.1 weight percent of chloride and nitrate, has the formula Ce(C₆H₅O₇), has greater than or equal to 10 weight percent water of hydration and is synthesized by combining cerium carbonate and citric acid to produce cerium citrate and carbon dioxide. 